1. Field of the Invention
The present invention relates to an optical scanner and an image forming apparatus, and more particularly to an optical scanner that scans a surface to be scanned with a light beam and to an image forming apparatus including the optical scanner.
2. Description of the Related Art
In a digital copier, a laser printer, a laser facsimile, and the like, an image is written using an optical scanner. Such an optical scanner includes a light source having a light emitter, a first optical system that forms an image of a light beam output from the light source as a long linear image extending in a main-scanning direction, a deflector having a deflecting reflective surface disposed near a position where the linear image is formed to deflect a light beam output from the first optical system, and a second optical system that condenses a light beam deflected by the deflector to a spot of light on a surface to be scanned, so that the surface is scanned with the light beam. A so-called multi-beam optical scanner in which a surface to be scanned is scanned with a plurality of light beams by using a multi-beam light source having a plurality of light emitters is also well known.
An increasing number of molded plastic products have come to be used for optical elements in the optical scanner, especially as a lens (scanning lens) used in the second optical system because the molded plastic products are economical and a free form surface can be achieved relatively easily. A molded plastic scanning lens is also positively adopted in the multi-beam optical scanner, in the same manner as in the conventional optical scanner having a single-beam light source.
In a molded plastic scanning lens, the refractive index distribution tends to be uneven.
To address this issue, Japanese Patent No. 3518765, for example, discloses a multi-beam optical scanner including a multi-beam light source, a first optical system that forms images of a plurality of light beams output from the light source as a plurality of long linear images extending in the main-scanning direction, an optical deflector that deflects the light beams, and a second optical system that condenses deflected light beams onto a surface to be scanned. The second optical system includes an optical element having an uneven refractive index distribution, and having a certain relationship between the number of multiple incident light beams, a pitch of chief rays of the multiple beams on a plane of incidence in a sub-scanning direction, the refractive index distribution, and an effective range of the lens corresponding to the effective write width on the surface to be scanned in the sub-scanning direction.
Furthermore, Japanese Patent Application Laid-open No. 2009-3393 discloses an optical scanner including, in the following order, a light source having a plurality of light emitters capable of performing independent optical modulation and arranged in a sub-scanning direction; an optical coupling element that converts a light beam output from each of the light emitters into a bundle of substantially parallel rays; an aperture for defining an outer edge of the parallel ray bundle; a collimating optical element that collimates the parallel ray bundle along the sub-scanning direction; a deflecting element that deflectively scans the ray bundle thus collimated; and a scanning optical system that forms images of the deflectively scanned ray bundle to scan a surface to be scanned. The scanning optical system has a plurality of lenses including resin lenses with positive power in the sub-scanning direction. The aperture and the resin lens are in optically conjugate relationship in the sub-scanning direction.
During a plastic molding process of an optical element, birefringence appears in a lens depending on its material, production conditions, its form, and other factors. Birefringence is a phenomenon where the refractive index becomes different for rays of light in directions perpendicular to each other, and is expressed by a main axis orientation and a phase difference. The main axis orientation herein has the same meaning as a fast axis orientation or a slow axis orientation.
Many scanning lenses are larger in size than pickup lenses (objective lenses), for example, used in an optical disk apparatus, and some molded plastic scanning lenses have an uneven birefringence distribution. In particular, a larger difference in thickness between the center and the peripherals of a lens, that is, a greater difference in thickness leads to more uneven birefringence distribution.
For example, it is assumed herein that, as illustrated in FIG. 25, two light beams (a beam 1 and a beam 2) output from different light emitters (ch1 and ch2) and separated from each other in the sub-scanning direction, pass through a scanning lens having a birefringence distribution illustrated in FIGS. 24A to 24C. In such a system, the birefringence of the scanning lens affects the beam 1 and the beam 2 differently. Therefore, as in an example illustrated in FIG. 26, the beam 1 and the beam 2, both of which are polarized linearly before being incident on the scanning lens, are polarized in a different manner after passing through the scanning lens. In FIG. 26, the beam 1 is elliptically polarized in a vertically elongated manner, and the beam 2 is elliptically polarized in a horizontally elongated manner. If a folding mirror is disposed between the scanning lens and the scanned surface, for example, because the reflectance of the beam 1 and that of the beam 2 are different on the folding mirror, the amounts of light on the scanned surface become different between ch1 and ch2. If the amounts of light on the scanned surface are different depending on the light emitters, the concentration of an output image might become uneven, and especially, banding might occur.
Furthermore, in a vertical cavity surface emitting laser array having a plurality of light emitters each outputting linearly polarized light, the direction of the polarity is rotated depending on the strength of the oscillation of the laser. The degree of the rotation differs in each of the light emitters (for example, see the paragraph [0003] in Japanese Patent Application Laid-open No. 2007-147864). If a vertical cavity surface emitting laser array having the light emitters outputting light with different polarization angles is affected by an uneven birefringence distribution, the difference in the amounts of light on the scanned surface is further increased, and the concentration of the output image would be more uneven, or banding would be more prominent.
Effects of an uneven refractive index distribution and an uneven birefringence distribution will now be explained. If a refractive index distribution is uneven, because the refractive power becomes different depending on a beam path, the light emitters have different focal points on the scanned surface, disadvantageously. This occurs only within a refractive optical system having a multi-beam light source and a scanning lens having an uneven refractive index distribution.
If the birefringence distribution is uneven, because the degree of birefringence becomes different depending on a beam path, the reflectance of the folding mirror and the transmittance of a dust preventing sheet glass, both of which are disposed between the molded plastic scanning lens and the scanned surface, become different for each of the beams. As a result, the amount of exposure on the scanned surface would be different for each of the light emitters, disadvantageously. This is caused by a multi-beam light source, a scanning lens having such an uneven birefringence distribution, and optical components (an optical reflecting member or an optical transmitting member) disposed between the scanning lens having an uneven birefringence distribution and the scanned surface. Such a system causes no difference in the amount of light on the scanned surface if no optical components are disposed between the scanning lens having an uneven birefringence distribution and the scanned surface.
In this manner, the effect of the uneven birefringence distribution differs from the effect of the uneven refractive index distribution.